Just yours truly to depend on (6)
I believe the answer is:
merely
'just' is the definition.
(synonyms)
'yours truly to depend on' is the wordplay.
'yours truly' becomes 'me' (the person who set this crossword).
'to depend' becomes 'rely' (to depend on someone is to rely on them).
'on' means one lot of letters go next to another.
'me'+'rely'='MERELY'
(Other definitions for merely that I've seen before include "Simply - only" , "Just one" , "'Only, and nothing more (6)'" .)